Key Structural Factors of Group 5 Metal Oxide Clusters for Base Catalytic Application [1st ed.] 9789811573477, 9789811573484

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Springer Theses Recognizing Outstanding Ph.D. Research

Shun Hayashi

Key Structural Factors of Group 5 Metal Oxide Clusters for Base Catalytic Application

Springer Theses Recognizing Outstanding Ph.D. Research

Aims and Scope The series “Springer Theses” brings together a selection of the very best Ph.D. theses from around the world and across the physical sciences. Nominated and endorsed by two recognized specialists, each published volume has been selected for its scientific excellence and the high impact of its contents for the pertinent field of research. For greater accessibility to non-specialists, the published versions include an extended introduction, as well as a foreword by the student’s supervisor explaining the special relevance of the work for the field. As a whole, the series will provide a valuable resource both for newcomers to the research fields described, and for other scientists seeking detailed background information on special questions. Finally, it provides an accredited documentation of the valuable contributions made by today’s younger generation of scientists.

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Shun Hayashi

Key Structural Factors of Group 5 Metal Oxide Clusters for Base Catalytic Application Doctoral Thesis accepted by The University of Tokyo, Tokyo, Japan

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Author Dr. Shun Hayashi The University of Tokyo Tokyo, Japan

Supervisor Prof. Tatsuya Tsukuda The University of Tokyo Tokyo, Japan

ISSN 2190-5053 ISSN 2190-5061 (electronic) Springer Theses ISBN 978-981-15-7347-7 ISBN 978-981-15-7348-4 (eBook) https://doi.org/10.1007/978-981-15-7348-4 © Springer Nature Singapore Pte Ltd. 2020 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Supervisor’s Foreword

Anionic metal oxide clusters called polyoxometalates (POMs), [MxOy]n– have been widely used as catalysts, especially for acid–base-catalyzed, oxidation, and photocatalytic reactions because of their unique and tunable chemical properties. Dr. Hayashi has gained interest in the application of POMs for base catalysts by taking advantage of the negative charge on the surface O atoms. He focused especially on POMs consisting of group 5 metals (Nb and Ta) as promising candidates for active base catalysts from the viewpoint of the trade-off between the basicity and stability of the surface O atoms under ambient conditions. His Ph.D. thesis aims to demonstrate a high potential of group 5 POMs as base catalysts and to improve the performance by elucidating the effects of structural factors, such as constituent metals, counter cations, and local structures of base sites. Brønsted and Lewis base catalysis for Knoevenagel condensation and CO2 fixation reactions, respectively, was studied while controlling the aforementioned factors systematically. A molecular-level understanding was provided about the effects of structural factors on catalysis by combined experimental and theoretical approaches. For example, he revealed experimentally that the key step of the CO2 fixation to styrene oxide on [Nb10O28]6– is a nucleophilic attack of adsorbed CO2 to epoxide. He proposed based on theoretical calculations that the nucleophilicities of adsorbed CO2 are determined by the coordination environment rather than the negativity of surface O atoms and that the O atoms at the corners acted as the most active sites. In addition, he found that the catalytic activity of [Nb10O28]6– was suppressed by using counter cations having shorter carbon chain due to the blocking of the base sites. This result suggests that a counter cation with a small charge density is suitable for the base catalytic application of POMs. Dr. Hayashi was awarded the CSJ Student Presentation Award in 2018 and the 8th Tokyo Conference on Advanced Catalytic Science and Technology (TOCAT8) Poster Prize in 2018 for his outstanding presentations. His Ph.D. thesis was awarded the School of Science Encouragement Award at the University of Tokyo in

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Supervisor’s Foreword

2019. I hope this book will provide rational design principles for base catalysts and lead to the development of innovative catalysts for solving energy and environmental problems. Tokyo, Japan May 2020

Prof. Tatsuya Tsukuda

List of Published Articles Parts of this thesis have been published in the following journal articles: 1. Shun Hayashi, Seiji Yamazoe, Kiichirou Koyasu and Tatsuya Tsukuda, “Application of Group V Polyoxometalate as Efficient Base Catalyst: a Case Study of Decaniobate Cluster”. RSC Adv. 2016, 6, 16239–16242, https://doi. org/10.1039/c6ra00338a. (Chap. 2) 2. Shun Hayashi, Seiji Yamazoe, Kiichirou Koyasu and Tatsuya Tsukuda, “Lewis Base Catalytic Properties of [Nb10O28]6– for CO2 Fixation to Epoxide: Kinetic and Theoretical Studies”. Chem. Asian J. 2017, 12, 1635–1640, https://doi.org/ 10.1002/asia.201700534. (Chap. 4) 3. Shun Hayashi, Naoto Sasaki, Seiji Yamazoe and Tatsuya Tsukuda, “Superior Base Catalysis of Group 5 Hexametalates [M6O19]8− (M = Ta, Nb) over Group 6 Hexametalates [M6O19]2− (M = Mo, W)”. J. Phys. Chem. C. 2018, 122, 29398–29404, https://doi.org/10.1021/acs.jpcc.8b10400. (Chap. 5) 4. Shun Hayashi, Seiji Yamazoe and Tatsuya Tsukuda, “Base Catalytic Activity of [Nb10O28]6–: Effect of Countercations”. J. Phys. Chem. C. 2020, 124, 10975– 10980, https://doi.org/10.1021/acs.jpcc.0c01488. (Chap. 3)

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Acknowledgements

The presented works were carried out under the supervision of Prof. Tatsuya Tsukuda. First of all, I would like to sincerely thank him for his guidance for as long as six years. His kind advices contributed to making significant progress in my thesis. He taught me the basic skills as a researcher from scratch. I have learned the way to think scientifically and the skills of presentations and writings. I hope and aspire to make the most of what I have learned from him in my career. Second to that, I am grateful to Prof. Seiji Yamazoe (Tokyo Metropolitan University), who was a former assistant professor of our group. He inspired me to take on this study. He especially taught me the characterization techniques by X-ray adsorption spectroscopy using synchrotron facilities. It will be a memorable reminiscence of the days and nights spent in the facilities during the measurement. Next, I owe my thanks to Associate Prof. Kiichirou Koyasu for his instructions in mass spectroscopy and theoretical calculations. Also, my heartfelt thanks go to Assistant Prof. Shinjiro Takano for every single piece of his keen insight into the experimental execution. Furthermore, I would like to convey my gratitude to Prof. Noritaka Mizuno (UTokyo) for undertaking as my sub-supervisor in Materials Education program for the future leaders in Research, Industry, and Technology (MERIT). Concurrently, I sincerely thank Prof. Kazuya Yamaguchi and Assistant Prof. Kosuke Suzuki (UTokyo) for giving me the opportunity to have beneficial discussion every three months. I appreciate the financial supports of MERIT program and Japan Society for the Promotion of Science (JSPS). Last but not least, I would like to express my heartfelt thanks to my friends and family for their selfless support over the three years. Shun Hayashi

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Contents

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1 1 1 4 8 8 10 14 16 18

2 Brønsted Base Catalysis of [Nb10O28]6− . . . . . . . . . . 2.1 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1 Electrospray Ionization Mass Spectrometry 2.1.2 X-Ray Absorption Spectroscopy . . . . . . . . 2.1.3 Density Functional Theory Calculations . . 2.2 Evaluation of Basicity Using DFT Calculations . . 2.3 Synthesis and Characterization . . . . . . . . . . . . . . 2.4 Brønsted Base Catalytic Activity . . . . . . . . . . . . . 2.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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3 Effect of Counter Cations on Base Catalysis of [Nb10O28]6− . . . 3.1 Decaniobate Clusters with Various Counter Cations . . . . . . . 3.2 Effect of Counter Cations on Brønsted Base Catalysis . . . . . 3.3 Poisoning by Counter Cations: Effect of Alkyl Chain Length 3.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1 Introduction . . . . . . . . . . . . . . . . . . . 1.1 Chemistry of Polyoxometalates . . 1.1.1 Structural Properties . . . . . 1.1.2 Group 5 Polyoxometalates 1.2 Catalysis by Polyoxometalates . . . 1.2.1 Acid Catalysis . . . . . . . . . 1.2.2 Oxidation Catalysis . . . . . 1.2.3 Base Catalysis . . . . . . . . . 1.3 Aim and Outline . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . .

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4 Effect of Local Structures on Lewis Base Catalysis of [Nb10O28]6− . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 CO2 Fixation with Styrene Oxide Catalyzed by [Nb10O28]6− . 4.2 Reaction Mechanism and Rate-Determining Step . . . . . . . . . 4.3 Determination of the Active Site . . . . . . . . . . . . . . . . . . . . . 4.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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5 Effect of Constituent Metals on Base Catalysis of Hexametalate Clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Synthesis and Characterization of Hexametalate Clusters . . . . 5.2 Brønsted and Lewis Base Catalysis . . . . . . . . . . . . . . . . . . . . 5.3 Activity Trend . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4 Effect of Protonation on Catalysis . . . . . . . . . . . . . . . . . . . . . 5.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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6 Concluding Remarks 6.1 Summary . . . . . . 6.2 Future Prospects . References . . . . . . . . .

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Curriculum Vitae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Chapter 1

Introduction

Abstract In this chapter, the structural and catalytic properties of metal oxide clusters [Mx Oy ]n− (M: metal), called polyoxometalates (POMs), are summarized. POMs have been applied as catalysts for acid/base-catalyzed, oxidation, photocatalytic, and electrochemical reactions by exploiting their controllable geometric structures and tunable chemical properties. Because POMs have atomically precise structures, a combination of experimental and theoretical studies has provided an understanding of how POMs function as catalysts and the key structural factors for catalysis. This chapter summarizes the previous work on the structural diversity of POMs, especially group 5 POMs, with a focus on the origin of their unique catalytic properties for acid/base-catalyzed and oxidation reactions. Finally, based on the trade-off between the stability and basicity of POM clusters, the possibility of using group 5 elements as the constituent metals of POMs for base catalytic applications is discussed. Keywords Polyoxometalate · Cluster · Structural factor · Catalyst

1.1 Chemistry of Polyoxometalates 1.1.1 Structural Properties Polyoxometalates (POMs), [Mx Oy ]n− , are anionic metal oxide clusters consisting of group 5 (V, Nb, Ta) or group 6 (Mo, W) elements [1–3]. The typical building block is a [MO6 ] octahedral unit, in which the metal atom is located at the center and the O atoms are located at the corners. POMs can be classified into two categories based on composition: heteropolyoxometalates and isopolyoxometalates. Heteropolyoxometalates contain internal heteroatoms in the form of [XO4 ] tetrahedral units (X = B, Al, Si, P, Ge, etc.). Typical structures are named after the researcher who discovered or predicted the structure, e.g., Keggin-type [XM12 O40 ]n− and Dawsontype [X2 W18 O62 ]n− structures. The Keggin-type [XM12 O40 ]n− structure contains an [XO4 ] unit surrounded by four [M3 O13 ] units, which can be rotated to form five structural isomers (α, β, γ , δ, and ε; Fig. 1.1a) [4–7]. The Dawson-type [X2 W18 O62 ]n− structure is a dimer of [XM9 O34 ] units, which can be regarded as a lacunary structure formed by removing one [M3 O13 ] unit from a Keggin-type structure (Fig. 1.1b) © Springer Nature Singapore Pte Ltd. 2020 S. Hayashi, Key Structural Factors of Group 5 Metal Oxide Clusters for Base Catalytic Application, Springer Theses, https://doi.org/10.1007/978-981-15-7348-4_1

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1 Introduction

(a)

(b)

[α-XW12O40]n – (d)

(c)

[X2W18O62]n – (e)

[α-XW11O39]n – (g)

[α-XMW11O39]n–

[W6O19]2 – (f)

[γ-XW10O36]n – (h)

[A-α-XW9O34]n – (i)

[γ-XM2W10O40]n–

[M4O2(XW9O34)2]n –

Fig. 1.1 Typical structures of POMs. a Keggin-type; b Dawson-type; c Lindqvist-type; d mono-; e di-; and f trivacant Keggin-type; g mono- and h dimetal-substituted Keggin-type; and i sandwich structure of trivacant Keggin-type. Color code: gray, W; pink, X; green, M; red, O

[8]. Isopolyoxometalates such as Lindqvist-type [M6 O19 ]n− structures contain no heteroatoms and are composed of a single type of metal atom. The Lindqvist-type [M6 O19 ]n− structure consists of six [MO6 ] units sharing their edges (Fig. 1.1c) [9]. The largest POMs reported are ring-shaped mixed-valence polyoxomolybdates named molybdenum blue or molybdenum brown [10]. A structural analysis was first reported for {Mo154 }, which has a ring topology [11, 12]. The largest POM ever characterized is {Mo368 }, which has a diameter as large as 5 nm [13].

1.1 Chemistry of Polyoxometalates

3

[MO4]n Low pH

High pH

Keggin-type [α-PW12O40]3

Lacunary POM [α-PW11O39]7

M-substituted POM [α-PMW11O40]4

Fig. 1.2 Typical synthetic procedure for lacunary and metal-substituted Keggin-type POMs. Color code: gray, W; pink, X; green, M; red, O

The synthesis procedures for POMs often consist of only a few steps. The key synthetic step is the acidification of the monomeric metalate precursor, as the nuclearity increases as the pH value of the reaction mixture decreases. The main synthetic parameters are as follows: (1) concentration/type of metal oxide anion, (2) pH, (3) ionic strength, (4) heteroatom type/concentration, (5) presence of additional ligands, (6) reducing agent, and (7) reaction temperature and processing method (e.g., microwave, hydrothermal, or refluxing) [3]. In addition to plenary structures, lacunary or metal-substituted POMs can also be synthesized [14]. Typical POMs are stable under low pH condition, but increasing the pH value of a plenary POM solution partially decomposes the structure, resulting in the formation of a lacunary structure. Lacunary structures are mainly observed for Keggin-type structures, with monovacant [XM11 O39 ]n− , divacant [XM10 O36 ]n− , and trivacant [XM9 O34 ]n− structures observed (Fig. 1.1d–f). These lacunary sites are good coordination sites for a variety of transition metals and can be used to form metal-substituted POMs. For Keggin-type structures, a vacant site can be introduced into the pristine cluster by increasing the pH value of the solution. Subsequently, a metal-substituted POM can be formed by decreasing the pH value of the solution in the presence of metal precursor ions (Fig. 1.2) [15]. A wide variety of metal-substituted POMs have been synthesized thus far by introducing transition metal such as V, Mn, Fe, Co, Ni, and Cu [16–21]. In the case of metal-substituted trivacant Keggin-type [XM9 O34 ]n− and Dawson-type [X2 W15 O56 ]n− structures, the substituted metals are sandwiched by two lacunary POMs to form dimers (Fig. 1.1g–i). As well as metal atom substitutions, the oxygen atoms of POMs can also be substituted by organic ligands (Fig. 1.3) [22]. The Lindqvist-type [Mo6 O19 ]2− structure has often been used as a platform, in which one of the terminal oxygen atoms can be substituted by an organic ligand (L) to form [Mo6 O18 (L)]n− . The terminal oxygen atom (O2− ) can be replaced by isoelectronic ligands including nitrido (N3− ) [23] and σ-donor/π-donor ligands such as imido (RN2− ) [24, 25], diazoalkane (R2 CNN) [26], and cyclopentadienyl (R5 C5 − ) groups [27, 28]. In addition, σ-donor/π-acceptor ligands are applicable, as observed for nitrosyl (NO+ ) [29] and diazenido (ArNN+ ) groups [30].

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1 Introduction

Fig. 1.3 Functionalization of terminal oxygen atoms in [Mo6 O19 ]2− . a [Mo6 O18 (N)]3− , b [Mo6 O18 {N(C6 H3 (CH3 )2 }]2− , c [Mo6 O18 {N2 C(C6 H4 -4-OCH3 )CH3 }]2− , d [Mo6 O18 {η5 C5 (CH3 )5 }]− , e [Mo6 O18 (NO)]3− , and f [Mo6 O18 (N2 C6 H5 )]3− . Color code: pink, Mo; red, O; blue, N; gray, C. Hydrogen atoms are omitted for clarity

1.1.2 Group 5 Polyoxometalates Among POMs containing group 5 elements (V, Nb, Ta), the chemical properties of polyoxoniobates (PONbs) and polyoxotantalates (POTas) are completely different from those of polyoxovanadates. Indeed, the chemical properties of polyoxovanadates are similar to those of group 6 POMs (Mo, W). Owing to synthetic difficulties, PONb and POTa chemistry has been less studied [31]. As discussed in Sect. 1.1.1,

1.1 Chemistry of Polyoxometalates Fig. 1.4 Geometric structures of a [W10 O32 ]4− and b [Nb10 O28 ]6− . Color code: gray, W; green, Nb; red, O

5

(a)

(b)

[W10O32]4 –

[Nb10O28]6 –

POMs consisting of V, Mo, and W can be synthesized by acidifying monomeric metalate [MO4 ]n− precursors. In contrast, owing to the absence of a stable and soluble precursor such as [MO4 ]n− , PONbs and POTas are synthesized by the decomposition of bulk metal oxides using strong bases such as alkali hydroxides. Differences in acid/base stability also exist as typical POMs consisting of V, Mo, and W are stable under low pH conditions (pH = 1–4), whereas PONbs and POTas are unstable under acidic conditions, immediately forming bulk oxides, but relatively stable under basic conditions. The first synthesis of a PONb, reported by Lindqvist for hexaniobate [Nb6 O19 ]8− [9] involved the calcination of niobium oxide (Nb2 O5 ) with an alkali hydroxide to form a reactive alkali niobate salt, followed by the addition of water to form the hexaniobate. Owing to the low reactivity of Nb2 O5 , hydrated niobium oxides or niobic acids (Nb2 O5 ·nH2 O) have received attention as alternative Nb sources for the solution synthesis of PONbs [32]. Decaniobate [Nb10 O28 ]6− was synthesized by Morosin et al. [33]. Although the number of constituent metal atoms is the same as in decatungstate [W10 O32 ]4− , the number of oxygen atoms and the geometric structure differ (Fig. 1.4). Notably, this cluster was synthesized using H2 O/MeOH as a solvent in the presence of tetramethylammonium (TMA) ions, showing the importance of nonaqueous solvents and organic counter cations. [Nb10 O28 ]6− has also been synthesized using a different organic cation, namely the tetrabutylammonium (TBA) ion [34]. Although the TMA salts of both [Nb6 O19 ]8− and [Nb10 O28 ]6− were formed when EtOH was used as a reaction solvent, they could be separated based on differences in solubility [35]. The structural diversity of PONbs was expanded by the synthesis of [SiNb12 O40 ]16− (Fig. 1.5a) and [H2 Si4 Nb16 O56 ]14− , as reported by Nyman et al. [36]. To enhance the reactivity of the niobate precursor, the synthesis was performed under hydrothermal conditions. This method enables the synthesis of clusters under stoichiometric conditions, whereas conventional synthesis processes require a large excess of base, which results in the dominant formation of [Nb6 O19 ]8− under high pH conditions. A third isopolyoxoniobate, [Nb20 O54 ]8− (Fig. 1.5b), was synthesized by the dimerization of [Nb10 O28 ]6− [37], and the geometric structure of [Nb20 O54 ]8− can be described as a dimer of [Nb10 O28 ]6− connected at two terminal oxygen atoms. In addition, a reversible structural change between [Nb10 O28 ]6− and [Nb20 O54 ]8− has been reported [34].

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1 Introduction

(a)

(b)

[SiNb12O40]16 – (d)

(c)

[Nb20O54]8– (e)

[Nb27O76]17– (g)

(f)

[Nb32O96]32– (h)

[TiNb9O28]7–

[Nb24O72]24 –

[Nb114O316]62 – (i)

[MnNb10O32]10 –

[Ti12Nb6O44]10–

Fig. 1.5 Typical structures of PONbs. a [SiNb12 O40 ]16− , b [Nb20 O54 ]8− , c [Nb24 O72 ]24− , d [Nb27 O76 ]17− , e [Nb32 O96 ]32− , f [Nb114 O316 ]62− , g [TiNb9 O28 ]7− , h [MnNb10 O32 ]10− , and i [Ti12 Nb6 O44 ]10− . Color code: green, Nb; blue, Si; light blue, Ti; purple, Mn; red, O

Recently, the application of [Nb6 O19 ]8− as a precursor for new types of PONbs has received considerable attention. Nyman et al. and Wang et al. independently reported the synthesis of [Nb24 O72 H9 ]15− (Fig. 1.5c) from [Nb6 O19 ]8− [38, 39]. This cluster has three [Nb7 O22 ] units connected by three [NbO6 ] units in an alternating fashion to form a ring structure. Cronin et al. reported the synthesis of [HNb27 O76 ]16− (Fig. 1.5d) and [H10 Nb31 O93 (CO3 )]23− by adding organic ligands [40]. The organic ligands were not included in the final structures but contributed to the formation of the clusters as structure-directing agents. Wang et al. later reported the synthesis of [H28 Nb32 O96 ]4− (Fig. 1.5e) composed of [Nb7 O22 ] units [41]. Zheng et al. reported the largest PONb, [Li8 Nb114 O316 ]54− (Fig. 1.5f), together with [Li3 KNb81 O225 ]41−

1.1 Chemistry of Polyoxometalates

7

and [H4 Nb52 O150 ]36− [42]. The building block of {Nb114 } can be viewed as {LiNb27 }, which is a structural analogue of {HNb27 } [40]. Four {LiNb27 } units are connected by two {Nb3 } units to form a central cavity that encloses four Li ions. These PONbs that are larger than [Nb6 O19 ]8− are synthesized using counter cations larger than alkali metal ions such as organic cations and copper complexes. The lower electron densities of such cations are essential for forming clusters with larger sizes. In contrast, alkali ions with higher electron densities prefer to form clusters with higher charge densities, such as [Nb6 O19 ]8− . As demonstrated for typical POMs, metal-substituted PONbs can also be synthesized. In such cases, bulk oxides and alkoxides act as the source of the substituent metals. For example, mono- and di-Ti-substituted PONbs, [TiNb9 O28 ]7− (Fig. 1.5g) and [Ti2 Nb8 O28 ]8− , which have the same geometric structure as [Nb10 O28 ]6− , have been synthesized by adding titanium alkoxide Ti(OR)4 to the reaction mixture [43, 44]. In these clusters, the central Nb atom(s) in [Nb10 O28 ]6− are selectively substituted by Ti atom(s). Casey et al. also reported the synthesis of a series of monometal-substituted decaniobates, [MNb9 O28 ]n− (M = Cr, Mn, Fe, Ni, Rh) [45– 47]. Nyman et al. reported the synthesis of a series of W-substituted hexaniobates [Nb6−x Wx O28 ](8−x)− (x = 2–4) [48] and Hu et al. reported a V-substituted decaniobate [V4 Nb6 O30 ]10− [49]. The introduction of a heterometal into the reaction mixture does not always result in metal substitution but instead may produce new types of geometric structures [50]. The introduction of Mn resulted in the formation of [H2 MnNb10 O32 ]8− (Fig. 1.5h), which has a similar geometric structure to [W10 O32 ]4– [51]. The introduction of Cr resulted in the formation of [CrIII 2 (OH)4 Nb10 O30 ]8− , which can be regarded as a dimer of Lindqvist-type [CrNb5 O19 ]n− [52]. Casey et al. reported the synthesis of [Ti12 Nb6 O44 ]10− with a hollow structure (Fig. 1.5i) [53]. Wang et al. reported the synthesis of [H6 Ge4 Nb16 O56 ]10− and [PNb12 O40 (VO)6 ]3− [54, 55]. In contrast to various metal-doped structures, lacunary structures have been limited to trivacant Keggin-type [(PO2 )3 PNb9 O34 ]15− , although the lacunary site was capped by three [PO4 ] units [56]. The chemical properties of Ta and Nb are similar because of their analogous electronic configurations. For example, the atomic radius of Ta (1.46 Å) is almost the same as that of Nb (1.46 Å) because of lanthanide contraction, and these elements can be mixed at any ratio, as observed for columbite (Fe,Mn)(Nb,Ta)2 O6 . However, the structural library of POTas is more limited than that of PONbs owing to the poorer reactivity of Ta precursors. Although Lindqvist-type [Ta6 O19 ]8− has been known since 1954 [57], the synthesis of a second POTa, [Ta10 O28 ]6− , was achieved in 2013 [58]. [Ta10 O28 ]6− , which has the same geometric structure as [Nb10 O28 ]6− and [V10 O28 ]6− , was synthesized through the dehydration condensation of [Ta6 O19 ]8− in an organic solvent. Recently, Casey et al. reported two POTas, [Ti2 Ta8 O28 ]8− and [Ti12 Ta6 O44 ]10− as structural analogues of [Ti2 Nb8 O28 ]8− and [Ti12 Nb6 O44 ]10− [59]. Hence, the structural discovery of POTas has just begun, and extended investigations are required to realize the subsequent development of PONbs. The synthetic methods for PONbs and POTas share similarities with those for POMs composed of V, Mo, and W in that structural controllability can be achieved

8

1 Introduction

by tuning the counter cation, solvent, and conditions. However, the group 5 POMs are synthesized from unreactive bulk oxides using strong bases. Despite this disadvantage, the structural diversity of PONbs has been expanded by using [Nb6 O19 ]8− as a precursor in the presence of unique counter cations, organic ligands, and surfactants under hydrothermal conditions.

1.2 Catalysis by Polyoxometalates Over the past five decades, the application of POMs as catalysts for acid-catalyzed, oxidation, photocatalytic, and electrochemical reactions has been extensively studied [60–69]. More recently, the base catalytic applications of POMs have been investigated [70]. As catalysts, POMs have the following advantages: (i) tunable chemical properties such as redox potential, acid–base nature, and geometric structure, (ii) thermal and oxidative stability, and (iii) active sites that are controllable through metal substitution. In addition, the catalytic performance of POMs can be studied both experimentally and theoretically at the molecular level owing to their atomically precise structures. In this section, the key structural parameters of POMs for catalysis are summarized.

1.2.1 Acid Catalysis POMs have historically been known as heteropoly acids owing to the strong acidity of their protonated forms. POMs have been applied as acid catalysts for alkene hydration, esterification, hydrolysis, and Friedel–Crafts reactions [60–62, 66, 69, 71]. Two factors can explain the acid catalytic performance of POMs: (i) the Brønsted acid strength and (ii) the soft basicity of the cluster anion. The acidity of POMs is simply correlated with the Brønsted acidity because of the weak basicity of POMs as counter ions to protons. As demonstrated in solid acids, the acidity can be estimated from the degree of interaction with basic probe molecules using the following techniques: (i) IR spectroscopy to observe the shifts in O–H stretching frequencies, (ii) 1 H NMR spectroscopy to observe the chemical shifts of 1 H peaks, (iii) the Hammett indicator method to observe the color changes of indicators, and (iv) the temperature-programmed desorption (TPD) of probe molecules [72]. It has been found that the acidities of POMs are stronger than those of the traditional metal oxides or typical inorganic acids such as H2 SO4 and HNO3 . The Hammett indicator method with various indicators having different pK a values revealed that the acidity of H3 [PW10 O40 ] is stronger than those of silica alumina (SiO2 –Al2 O3 ) and protontype zeolite (H-ZSM-5) [73]. The order of acidity for various Keggin-type POMs, as estimated via 1 H NMR analysis based on the strength of the hydrogen bond between the probe molecule and POMs, is as follows [74]:

1.2 Catalysis by Polyoxometalates

9

H3 [PW12 O40 ] > H3 [PMo12 O40 ] > H4 [SiW12 O40 ] > H4 [SiMo12 O40 ] >> H2 SO4 , HNO3 Another important parameter of POMs for the acid catalysis is the soft basicity of the cluster anion. Izumi et al. estimated the order of softness for POMs using the equilibrium constant for ion exchange between silver iodide and the sodium salts of POMs: AgI + Nan X  AgNan−1 X + NaI

(1.1)

where Xn− represents a POM [75]. As Ag+ is a softer acid than Na+ , a larger equilibrium constant indicates a softer basicity. The estimated order of softness is as follows: [SiW12 O40 ]4− > [PW12 O40 ]3− > [PMo12 O40 ]3− 2− > [SiMo12 O40 ]4− > NO− 3 > SO4

The authors proposed that soft basicity contributes to the stabilization of cationic intermediates. The catalytic activities for ether cleavage reactions decrease in the order of H3 [PW12 O40 ], H4 [SiW12 O40 ] >> H3 [PMo12 O40 ], and this trend has been explained based on both the Brønsted acidity and the soft basicity of the cluster anions [75]. The protonation sites of POMs, which are expected to be the most basic O atoms on the surface, can be located using crystallography techniques and theoretical calculations. There are two types of surface O atoms in POMs: bridging (μ2 -O, M–O–M) and terminal (η-O, M=O) sites. As bridging O atoms are in general more basic than terminal O atoms, the most probable protonation sites of POMs are the bridging O atoms [76–78]. The internal O atoms, which are coordinated to more than three metal atoms, are not typically accessible, except in a few particular cases such as Anderson and Preyssler structures. For Preyssler structures, the protonation site has been determined to be the internal O atoms [79]. Theoretical calculations have played a significant role in understanding acid catalysis by POMs. Iglesia et al. proposed the deprotonation energy (DPE) as a key descriptor of the acidity [80]. The DPE is defined as the energy required to remove a proton from a neutral cluster to form an anionic conjugated base (HA → H+ + A− ). Therefore, a low DPE value implies strong acidity. The authors found a good correlation between DPE and the experimental kinetic parameters obtained for various acidcatalyzed reactions including alcohol dehydration, ether cleavage, alkane isomerization, and alcohol condensation. The calculated DPEs of four Keggin-type structures, H8−n [Xn+ W12 O40 ] (Xn+ = P5+ , Si4+ , Al3+ , Co2+ ), were found to decrease with an increase in the oxidation state of the central atom, indicating that the acidity increases in the order as the oxidation state: H6 [CoW12 O40 ] < H5 [AlW12 O40 ] < H4 [SiW12 O40 ] < H3 [PW12 O40 ] [80]. This result is consistent with the order of Brønsted acidity observed experimentally.

10

1 Introduction

(a)

(b) A- + H+ + R -PA

A- + RH+ Δ Eint

Energy

DPE

[A-…RH+] AH + R Δ Hads

Ea=DPE-PA+Δ Eint-Δ Hads AH…R

Fig. 1.6 a Proposed sequence of elementary steps for 2-butanol dehydration on H3 [PW12 O40 ]. Reprinted from Ref. [80] with permission. 2007, John Wiley and Sons. b Thermochemical cycle for acid–base reactions on Brønsted acid catalysts. Reprinted from Ref. [82] with permission. 2009, Elsevier Ltd.

However, the reaction rate and selectivity of acid-catalyzed reactions cannot be always explained by the DPE. For example, Iglesia et al. tested the catalytic activity of Keggin-type POMs for 2-butanol dehydration [81]. A kinetic study and theoretical calculations showed that the dehydrogenation proceeds via E1 elimination, with the key step of the reaction being the formation of a carbenium intermediate (Fig. 1.6a). Therefore, the activation energy (E a ) of the reaction can be written as follows: E a = DPE − PA + E int − Hads

(1.2)

where PA, E int , and H ads represent the proton affinity of the reactant, the ion-pair stabilization energy (carbenium intermediate and POM), and the adsorption energy of the reactant, respectively (Fig. 1.6b) [82]. As PA and H ads are not strongly correlated to the nature of POMs, E a is determined by the sum of DPE and E int . This result indicates that the soft basicity of POMs is as important as the Brønsted acidity (DPE) for stabilizing the cationic intermediate (E int ). Ion-pair stabilization has also been reported to be significant for other reactions in which the key step is the formation of a carbenium ion via E1 elimination [83–85].

1.2.2 Oxidation Catalysis POMs have been applied as catalysts for oxidation reactions such as alkene epoxidation, light alkane oxidation, arene oxidation to form phenol derivatives, and organosilane oxidation [60–62, 66, 67, 69, 86]. This section focuses on three POMs that have been applied as catalysts for alkene epoxidation as examples for understand the oxidation activity at the molecular level: (i) the practically and historically important [WO4 ]2− /[PO4 ]3− system; (ii) the recently developed, efficient [γ-H4 SiW10 O36 ]4−

1.2 Catalysis by Polyoxometalates

11

system; and (iii) a series of monometal-substituted Keggin-type [M(L)PW11 O39 ]n− structures.

1.2.2.1

[WO4 ]2− /[PO4 ]3− System

In 1983, Venturello et al. first reported the selective epoxidation of alkenes with H2 O2 to form epoxides using the [WO4 ]2− /[PO4 ]3− system [87]. The formed epoxides easily underwent hydrolysis to diols in the presence of water. Therefore, this reaction was carried out in a biphasic system, in which the active species generated in aqueous layer was transferred to the organic layer using a phase transfer reagent. During the reaction, [WO4 ]2− and [PO4 ]3− were found to form a peroxo intermediate, [PO4 [WO(O2 )2 ]4 ]3− , which was isolated and characterized crystallographically (Fig. 1.7a) [88]. The isolated peroxo complex was active for the reaction, suggesting that the complex was responsible for the catalytic activity in the system. Ishii et al. used H3 [PM12 O40 ] (M = Mo, W) to catalyze the epoxidation of alkenes with H2 O2 and later found that the active species was also [PO4 [MO(O2 )2 ]4 ]3− generated by the decomposition of [PM12 O40 ]3− [89, 90]. Xi et al. remarkably improved this system by applying [π-C5 H5 NC16 H33 ]3 [PO4 (WO3 )4 ] as a heterogeneous catalyst [91]. The reaction was carried out in an organic solvent in which the catalyst was insoluble. The key to this system was that insoluble [PO4 (WO3 )]3− was converted into soluble [PO4 [WO(O2 )2 ]4 ]3− in the presence of H2 O2 and that the catalyst was reprecipitated when H2 O2 was consumed.

1.2.2.2

[γ -H4 SiW10 O36 ]4− Cluster

In 2003, Mizuno et al. reported that [γ -H4 SiW10 O36 ]4− prepared by protonating divacant Keggin-type [γ -SiW10 O36 ]8− at a pH value of 2 showed excellent catalytic activity for the epoxidation of various alkenes with H2 O2 [92, 93]. Advantageously, this catalyst exhibited ≥99% selectivity for the epoxide and ≥99% efficiency of H2 O2 utilization, even at 305 K. Active electrophilic intermediates were reported to form during the reaction, which were responsible for the observed induction period [94]. However, the induction period disappeared following the pretreatment of [γ SiW10 O36 ]8− with H2 O2 , and diperoxo species [γ -SiW10 O32 (O2 )2 ]4− was found to be generated during the induction period. However, isolated [γ -SiW10 O32 (O2 )2 ]4− was inactive for this reaction, suggesting that secondary species were formed in the presence of H2 O2 during the reaction (Fig. 1.7b). As unprotonated [γ -SiW10 O36 ]8− is inactive in the reaction, the protonation of the cluster is the key in this system. Two isomers were proposed for the protonated [γ -H4 SiW10 O36 ]4− species: (i) [γ -H4 SiW10 O34 (H2 O)2 ]4− with two water ligands and two oxo ligands and (ii) [γ -H4 SiW10 O32 (OH)4 ]4− with four hydroxy ligands (Fig. 1.8). Mizuno et al. confirmed that the addition of two equivalents of base with respect to [γ -H4 SiW10 O36 ]4− inhibited the catalytic reaction and suggested the formation of the first isomer [94]. Using NMR analysis, Bonchio et al. found that only

12 Fig. 1.7 a Geometric structure of [PO4 [WO(O2 )2 ]4 ]3− [88]. b Proposed reaction mechanism for the epoxidation of alkenes catalyzed by [γ -H4 SiW10 O36 ]4− . Reprinted from Ref. [94] with permission. 2007, John Wiley and Sons

1 Introduction

a

b

Fig. 1.8 Schematic views of two possible protonation patterns for [γ -H4 SiW10 O36 ]4− . a [γ H4 SiW10 O34 (H2 O)2 ]4− and b [γ -H4 SiW10 O32 (OH)4 ]4− [95]. Color code: gray, W; blue, Si; red, O; light blue, H

1.2 Catalysis by Polyoxometalates

13

two protons in [γ -H4 SiW10 O36 ]4− are reactive to base [95], which implied that only two protons in [γ -H4 SiW10 O36 ]4− contribute to promoting the catalytic reaction. In contrast, Musaev et al. proposed the second isomer based on a computational study [96]. The authors also theoretically predicted the reaction mechanism, in which the first step is the formation of hydroperoxide (–OOH) by the reaction of a hydroxyl group with H2 O2 and the second step is oxygen transfer to an alkene to form an epoxide [97].

1.2.2.3

Monometal-Substituted Keggin-Type [M(L)PW11 O39 ]n− System

Monometal-substituted Keggin-type POMs [α-XMW11 O39 ]n− (X = P, Si; M = Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, V, Nb, etc.) and their dimers have been applied as catalysts for alkene epoxidation [98–101]. Disubstituted Keggintype POMs [γ -XM2 W10 O38 ]n− [102–104] and sandwich-type POMs such as [ZnWM2 (ZnW9 O34 )2 ]n− [105–107] are also known oxidation catalysts. The key step of the catalytic reaction is regarded to be the formation of a peroxo- and/or hydroperoxo-type intermediate around the metal-substituted site. In this section, oxidation catalysis by metal-substituted POMs is explained using Ti-substituted Keggin-type [α-PTiW11 O40 ]5− as an example. [α-PTiW11 O40 ]5− has been extensively studied as a model single-site catalyst for titanium silicate [108], which is known to be an efficient heterogeneous catalyst among solid metal oxides for the selective epoxidation of alkenes [109, 110]. In H2 O2 -based systems, hydroperoxotitanium (Ti-OOH) is the accepted active species rather than a peroxo species. It was revealed that isolated peroxo species [PW11 (TiO2 )O39 ]5− [111] is inactive for the epoxidation reaction and that the reaction rate is strongly influenced by the formation rate of Ti-OOH (Fig. 1.9) [112]. The protonation of the O atoms adjacent to the Ti atom has a significant influence on the selectivity of the reaction. It was reported both experimentally and theoretically that the addition of one equivalent of acid to [PTiW11 O40 ]5− reduces the activation energy of alkene epoxidation [113]. However, in the case of the alkene oxidation catalyzed

Fig. 1.9 Schematic representation of the mechanism for alkene epoxidation with H2 O2 by a Tisubstituted POM. Reprinted from Ref. [113] with permission. 2012, The Royal Society of Chemistry

14

1 Introduction

by Hn [PTiW11 O40 ](5−n)− (n = 1–5), the selectivity for epoxides became low when n ≥ 2 [114]. These results suggest that there is an optimal degree of protonation that allows the formation the Ti-OOH intermediate, whereas excessive protonation enhances the electrophilicity of the O atoms, resulting in a decrease in the selectivity for epoxides. In summary, the application of POMs as oxidation catalysts began with the discovery of peroxo compound [PO4 [WO(O2 )2 ]4 ]3− as an active species. With [γ H4 SiW10 O36 ]4− , which displayed excellent catalytic activity, the formation of a peroxo species was also the key for the catalytic reaction. Furthermore, with metalsubstituted POMs such as [PTiW11 O40 ]5− , the oxidation catalytic performance was explained by the formation of peroxo- and/or hydroperoxo-type intermediates on the substituted metal.

1.2.3 Base Catalysis Compared with the catalytic applications to oxidation and acid-catalyzed reactions, the base catalytic applications of POMs have scarcely been reported. In the case of typical solid bases such as metal oxides and zeolites, the base properties, including number of sites, local structure, and base strength, are not uniform. Therefore, the base properties of solid bases are determined as an average over the surface using conventional characterization techniques including the indicator method, spectroscopy using probe molecules, and TPD [115]. In contrast, the base properties of POMs can be discussed with atomic precision based on the cluster structures. The base sites of POMs are the surface O atoms, and the negative charge densities of the surface O atoms can be viewed as a key factor for base catalysis. The negativities of the surface O atoms can be evaluated experimentally according to the position of the protonated O atoms determined by X-ray crystallographic analysis and NMR analysis. Theoretical calculations are also helpful in determining the base sites via molecular electrostatic potential maps and the stabilization energies of protonation. For Lindqvist-type [M6 O19 ]n− , the surface O atoms are divided into two groups: bridging (μ2 -O) and terminal (η-O) sites. The bridging O atoms are generally regarded as the most basic sites, as demonstrated crystallographically for [Mo6 O19 ]2− [77] and [H2 Ta6 O19 ]6− [116] and predicted theoretically for [W6−x Nbx O19 ](2+x)− [78]. In the case of [V10 O28 ]6− , the base sites were found to be facet (μ3 -O) and bridging (μ2 O) sites based on the protonation sites of [H3 V10 O28 ]3− together with theoretical calculations [76]. Over the last several years, the application of POMs as base catalysts has been investigated by Mizuno, Kamata et al. [70]. They focused on the possibility of monomeric tungstate, [WO4 ]2− , as a base catalyst [117–119]. As [WO4 ]2− has a high charge density (negative charge per cluster size), the basicity of the O atoms was expected to be high. The authors estimated the basicities of the surface O atoms in various tungstates using density functional theory (DFT) calculation and natural bond orbital (NBO) analysis. The NBO charge (−0.934) of an O atom in [WO4 ]2−

1.2 Catalysis by Polyoxometalates

15 – 0.652

– 0.569 – 0.934

– 0.721

– 0.730 – 0.592 –0.722

– 0.584 – 0.728

– 0.753

– 0.744

[WO4]2 –

[W6O19]2–

[W10O32]4 –

[α-SiW12O40]4–

Fig. 1.10 Geometric structures of various POMs showing the NBO charges of the surface O atoms [117]

is lower than those in other tungstates such as [W6 O19 ]2− , [W10 O32 ]4− , and [αSiW12 O40 ]4− (−0.569 to −0.753), suggesting that [WO4 ]2− acts as a strong base catalyst (Fig. 1.10). The authors demonstrated that it acted as an efficient catalyst for CO2 fixation reactions with various amines. The catalytic activity of [WO4 ]2− was ascribed not only to the basicity of the surface O atoms but also bifunctional catalysis, in which both CO2 and amines were activated. The hybrid system of Rh2 (OAc)4 and TBA2 [WO4 ] was also developed [120, 121]. This concept was expanded to the more negatively charged monomeric anion [PO4 ]3− , the base catalytic activity of which was ascribed to the high charge density of the monomeric anion [122]. Lacunary POMs were also utilized as base catalysts by increasing the negative charge in the cluster through the introduction of a lacunary site. A lacunary tungstogermanate, [γ -Hn GeW10 O36 ](8−n)− (n = 1, 2), showed base catalytic activity in the Knoevenagel condensation of benzaldehyde with nitriles with various pK a values [123, 124]. The pK a values of the nitriles that can be coupled with the aldehyde act as an indicator of the basicity of the catalysts. For the reaction to proceed using nitriles with weak acidity (large pK a value), catalysts with strong basicity are required. The reaction proceeded with [γ -H2 GeW10 O36 ]6− when nitriles with pK a values of less than 21.9 were used. This result is in contrast to the reports that [W6 O19 ]2− , [γ -H4 SiW10 O36 ]4− , [H(γ -SiW10 O32 )2 (μ-O)4 ]7− , and [Si2 W18 O62 ]8− could catalyze the reaction with malononitrile and ethyl cyanoacetate, which have low pK a values of 11.1 and 13.1, respectively [125–127]. Moreover, the catalytic activity was significantly increased with the increase in the number of negative charges in the cluster from −6 to −7 caused by removing a proton from [γ -H2 GeW10 O36 ]6− . This observation implies that the charge density of the cluster has an important effect on enhancing the basicity of the surface O atoms. To apply POMs as homogeneous base catalysts, organic cations such as TBA ions are used as counter cations in organic solvents. The heterogeneous base catalytic activity of Na8 [HPW9 O34 ] using alkali metal ions such as Na+ as counter cations has also been reported [128]. Despite the high charge density (−8) of the cluster, base catalytic activity has only been reported for nitriles with low pK a values (